High-efficiency Step-Up Converter by using PV system with Reduced Diode Stresses Sharing

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal, Volume 4, Issue 2, February 2014)

319

High-efficiency Step-Up Converter by using PV system with

Reduced Diode Stresses Sharing

V. Thirumurugan M.E., (PhD).

1

, S.Sugumar B.E., (M.E).

2

1

AP, EEE, The Kavery Engineering College, Salem, Tamilnadu, India

2_{PG Scholars, EEE, The Kavery Engineering College, Salem, Tamilnadu, India}

Abstract- High-efficiency Step-Up Converter by using PV system with Reduced Diode Stresses sharing is proposed in this paper. Through employ of coupled inductor and switched capacitor, the proposed converters attain high step-up conversion ratio without operating at extreme duty ratio. Due to reutilize of leakage energy, the efficiency is developed and the large voltage spike on switch is improved, This kinds a low-voltage-rated MOSFET with low RDS(ON) can be implemented for decreases of conduction losses .Now adding, the selection of diodes is useful and inexpensive because all the reduced voltage strains on diodes are the same. Finally, a 700 W prototype circuit with 12 V input and 400 output V output is employed to prove the performance, and the maximum efficiency is 99%.

Keywords- Coupled inductor, Switched Capacitor, high step-up converter.

I. INTRODUCTION

Renewable energy systems commonly make low Voltage output, hence, high step-up dc/dc converters have been usually valued and working in many renewable energy applications [1]-[5]. Such systems transfer energy from renewable sources into electrical energy, and convert low voltage into high voltage via a step-up converter, which can convert energy into electricity using a DC-Micro grid. Therefore, a high step-up converter achieves highly between the systems because such system requires a suitably high step-up conversion with high efficiency. Fig. 1 shows a typical renewable energy system. Due to leakage inductance, conventional step-up converters, such as the boost converter and fly back converter, cannot attain a high step-up conversion with high efficiency. So, various high step-up converters with methods of coupled inductor and switched capacitor, which have features of leakage energy reutilize, low voltage stress, and high efficiency are proposed [6]-[10].

Fig.1.Typical renewable energy system

II. PROPOSED CONVERTERS AND OPERATING

PRINCIPLES

The circuit configurations of the proposed converters are shown in Fig. 2. The proposed converter I and proposed converter II have same the number of apparatuses, step-up gain and voltage stresses. The proposed converter III is integrated and derived from the both proposed converters I and II.

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Fig.2.The proposed step-up converters. (a) Converter I. (b) Converter II. (c) Converter III.

The primary winding and secondary winding of coupled inductor have Np turns and Ns turns respectively, and n is equal to Ns/Np, which is defined as turns ratio and can be controlled to extend step-up gain. In primary side, Lm and Lkp denote the magnetizing inductor and leakage inductor respectively, and the secondary leakage inductor is Lks. The boosting capacitor and switched capacitor are denoted as Cb and Cs, respectively. In the circuit analysis, the simulation results of proposed converters I and II are almost near apart from the voltage on capacitors. The proposed converter III is the main importance of operating analysis, which is operated in continuous conduction mode (CCM).The key steady waveform as well as operating modes of proposed converter III, are plotted in Fig. 3 and Fig. 4 respectively.

Fig.3.Steady waveform of the proposed converter III in CCM.

(a)

(b)

(c)

(d)

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Fig.4.The operating modes of the proposed converter III. (a) Mode I [to, t1]. (b) Mode II [t1, t2]. (c) Mode III [t2, t3]. (d) Mode IV [t3, t4].

(e) Mode V [t4, t5]. (f) Mode VI [t5, t6].

Mode I [t0, t1]

At t=t0, the power switch S starts to turn on, and diodes Ds1 and Ds2keep on reversed-biased state, as shown in Fig. 4(a). The magnetizing inductor Lm still transfers energy to the boosting capacitors Cb1 and Cb2. Also, the high voltage energy still provides to output terminal

Mode II [t1, t2]

At t=t1, all semiconductor devices continue before state, as shown in Fig. 4(b).The output capacitor Co variations from discharging state to charging state, this mode is related to before mode.

Mode III [t2, t3]

At t=t2, all diodes are reversed-biased state apart from diode Ds2, as shown in Fig. 4(c).The magnetizing current and primary leakage current increase linearly. The switched capacitor Cs2 boosts the voltage from the induced voltage of secondary winding and the boosting capacitor Cb2, which kinds the switched capacitor Cs2capable to have a high voltage.

Mode IV [t3, t4]

At t=t3, the diodesDs1 and Ds2 are forward-biased state, as shown in Fig. 4(d).The switched capacitor Cs2arises to boost the voltage from the induced voltage of secondary winding, the boosting capacitor Cb1 and input voltage source Vi, which kinds the switched capacitor Cs1capable to have a high voltage.

Mode V [t4, t5]

At t=t4, the power switch S activates to turn on, and the diodes Ds1 and Ds2turn off certainly, as shown in Fig. 4(e). For the meantime, the primary and secondary windings of coupled inductor are consistently connected in series, and the magnetizing inductor Lm starts to transfer energy to the boosting capacitors Cb1 and Cb2.

Mode VI [t5, t6]

At t=t5, the output diode Do activates to turn on, and the output capacitor Co get replacement, as shown in Fig. 4(f). For the meantime, input voltage source Vi, coupled inductor, switched capacitors Cs1 and Cs2 provide energy with high voltage gain to output capacitor Co and load Ro.

III. STEADY-STATE ANALYSIS

The transient characteristics of integrated circuit are marginalized to make simpler the circuit performance analysis of the proposed converter in CCM, and some formulated assumptions are as follows:

(1) All of the components in the proposed converter are ideal.

(2) Leakage inductors Lkp and Lks are ignored.

(3) Voltages on all capacitors are measured to be constant because of substantially large capacitance.

By applying the voltage-second balance to the magnetizing inductor based on KVL, the voltage gains and voltage stresses of proposed converters can be calculated.

A. Voltage Gain

The voltage gains of proposed converters I and II are the same and all of voltage gains can be given by

D

n

II

MV

I

MV







1 )

1 (

2 )

(

)

(

(1)

D

n

III

MV







1 )

2 (

3 )

(

(2)

Equation (1) and (2) confirms that the proposed converters possess high step-up voltage gains without an extreme duty cycle. The curve of all the voltage gains related to duty cycle lower than turns ratio n= 2, is shown in Fig. 5.

B. Voltage Stress

The voltage stresses on semiconductor devices of proposed converter I are equal to proposed converter II. All of switch stresses can be given by

)

1 (

2

1 )

(

)

(

D

n

Vo

D

Vi

II

VS

I

VS









(3)

)

2 (

3

1 )

(

D

n

Vo

D

Vi

III

VS







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Fig.5.The voltage gains versus duty cycle under n = 2.

Equations (3) and (4) approve that low-voltage-rated MOSFET with low RDS(ON) can be implemented for the proposed converters to decrease conduction losses. All of diode stresses of any proposed converter are equal to each other and can be given by

Vo

D

n

Vi

D

n

II

all

VD

I

all

VD

)

1 (

2

1

1 )

(

,

)

(

,











(5)

Vo

D

n

Vi

D

n

III

all

VD

)

2 (

3

1

1 )

(

,











(6)

Equations (5) and (6) denote that all of diode stresses of proposed converters are considerable lower than output voltage, and the diode stresses of any proposed converter get reduced and usual. The correlation between all the comparative voltage stresses and the turns ratio n

lower than duty cycle D = 0.5 is showed in Fig. 6.

Fig.6.The relative voltage stresses versus turns ratio lower than D = 0.5.

IV. SIMULATION MODEL

The simulation model of step-up converter with fuzzy controller as shown in fig.7 the two inputs like radiation and temperature are given to solar PV system.

Its converts the heat energy to electrical energy means DC voltage. This electrical voltage and current are viewed by both voltage and current measurement separately. This electrical signal is given to DC – DC converter. The MOSFET also connect to this section to improve the efficiency of the converter. The converter converts the low level signal to required level with pure DC signal. The coupled inductor is used to increase the voltage level and capacitor is mainly filtering the DC signal. This pure DC signals are very high efficiency and these are measured by both voltage and current measurement and viewed by scope. The part of output signals are given to fuzzy logic controller and rule viewer. This section contains some standard reference values and rules. The incoming output signal and reference signal are compared and produce the difference output is given to PWM generator. The PWM generator is convert the output value of fuzzy controller to pulse signal and given to MOSFET. Depending upon pulse signal the MOSFET is switched the electrical signal and the high efficiency of pure DC voltage. In this system is closed loop system of step-up converter with fuzzy controller.

Fig.7.Simulation model of step-up converter with fuzzy controller

V. SIMULATION RESULTS

A 700 W prototype of the proposed high step-up converter III is verified. The electrical specifications are Vi = 12 V, Vo = 400 V, fs = 45 kHz. The major components have been selected as magnetizing inductors Lm = 11µH, turns ratio n = 6.8, power switches SisBUK965R8-100E, all diodes are MBR10200FCT, capacitorCb1, Cb2 and Cs2= 22 µF, Cs1 = 56µF, input filter capacitors = 2200µF * 2, output filter capacitor =82µF.

ra1

Discrete, Ts = 5e-005 s.

powergui v + -Vo1 v + -Vo 25 T i + -Scope4 Scope3 Scope2 Scope12 Scope11 Scope10 Scope1 Scope TPulses SVPWM Generator Product2 Product1 Product

Parallel RLC Branch1

? ? ? PV System1

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Fig.8 shows the measured waveforms at full load 700W. Fig.6.4 (a) shows the PWM signals Vgs and the voltage stress on the power switch, as well as the current through the primary leakage. The switch stress much lower than output voltage (400V) show the voltage stresses on all diodes and output voltage Vo, and all of the diode stresses are properly 178V which are lower than the half of output voltage (400V). From the experimental results, the per voltage stress, reduced voltage stresses step-up conversion are effectively implemented.

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(b)

(c)

Fig.8.The measured waveforms at full load 700 W

From the proposed system, step-up converter without fuzzy controller gives desired output ranges.

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(b)

(c)

Fig.9.Output Waveform

VI. CONCLUSION

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For the meantime, the voltage stress on the power switch is controlled and much lower than the output voltage (400 V). Finally a 12-V input and 400-V output voltages is implement and test to show the efficiency via MAT LAB simulation. Furthermore, the highest efficiency is 99% at Po= 700 W. Thus, the proposed converter is suitable for high step-up conversion applications.

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